Telephone system



Get. l, 1946.. R, 0, W|sE 2,408,452

TELEPHONE SYSTEM Filed June 8, 1945 3 Sheets-Shea?l 1 Oct. 1, 1946'. R'. o. wlsE 2,408'452 TELEPHONE SYSTEM Filed June 8, 1945 5 Sheets-Sheet 2 R. o. wlsE 2,408,462 TELEPHONE SYSTEM Filed June 8, 1943 3 Sheets-Sheet 3 ATTORNEY Oct. 1, 1946.

W MUO Patented Oct. 1, 1946 21,408,462 TELEPHONE SYSTEM Raymond 0. Wise, Short Hills, N. J., assignor to Bell Telephone Laboratories, Incorporated, New

York, N. Y.

VApplication June 8, 1943, Serial No. 490,033

Claims. (Cl. l79--l5) This invention relates to telephone systems I and particularly to systems in which connections between calling and called lines are established by subscriber controlled automatic selection of idle channels through multichannel communication media.

Among the objects of the invention are the provision of a more efcient and reliable common medium multichannel exchange and the provision therein of the large number of channels necessary in an automatic telephone system without the use of expensive Variable frequency lters.

This invention is an automatic telephone system in which connections between calling and called lines are automatically completed under the control of the calling subscriber over channels of multichannel transmission media, one channel being used for each direction of conversation. to constitute a conversational connection, a feature of the invention being a trunking arrangement wherein a fixed band-pass lter is provided for the demodulator of each channel, carrier sources of required frequency are selectively associated with the modulators of the channels and a pentode amplier is provided for each modulator to prevent interference between channels.

Another feature of the invention is atrunk` ing arrangement including channels of a common medium wherein carrier sources of required frequency areselectively connected tothe modulators, isolating resistances being provided for each modulator to prevent cross-talk between channels, the number of modulators connected in parallel to a group or line amplifier being large enough to prevent interference between channels. A further feature is av trunking arrangement including the channels of a common medium wherein carrier Sources of required frequency are selectively connectedV to the modulators, isolating resistances-'are provided for each modulator to prevent cross-talk between channels, group filters are provided for groups of channels, a carrier source of a different frequency is individuallyr associated with each demodulator, and a channel amplifier is provided for each demodulator to obtain satisfactory volume. 4

X A further feature is a carrier switching system for establishing connections between calling and called telephone lines under the control of the calling subscriber in which a double stage of modulation is employed to obtain single lsideband operation, carrier sources of different frequencies are individually connected to :first modulators of the channels, fixed band-pass lters interconnect the rst and second modulators of the channels, carrier sources of required frequencies are selectively connected to the second modulators of the channels, and carrier sources of different frequencies are individually connected to the demodulators.

These and other features of the invention are embodied inthe systems shown schematically in the drawings which form a part of this specification.

Referring to the drawings: i

Figs. l and lA show a telephone system comprising telephone offices A and B interconnected by multichannel transmission media VCMII, CMIZ, CMI3 and CMM;

Fig. 2 shows a telephone system comprising offices C and D interconnected by multichannel transmission media CM2I, CM22, CMZS and CM24; and

Fig. Bishows aV telephone system comprising telephone oliices E and F interconnected by multichannel transmission media CM3I, CM32, CM33 and CMM. y

Consider first the system represented in Fig. 1. The subscribers stations SI, S2, S3 and S4 are of the usual type connected to lines terminating in common battery automatic telephone o'ces and each of the stationsincludes a dial or other impulse sending device for controlling the establishment of` desired connections.V The line switches LSI, LS2 andV final switches FSI and FS2 may be of the crossbar type, the line switches being operated in responsejto the-initiation of calls at subscribers stations to connect calling lines to district circuits and the final switchesbeing directively controlled by ,sendermarker means to select calledA lines. Reference may be had to the patent to W. W. Carpenter 2,093,117, granted September 14,- 1937, for a detailed description of the operations `involved in connecting .calling lines to district circuits and the operations of final switches in connecting with called lines in a crossbar system. The line switches LSI and L52 may be similar to the line switch shown in Figs. 14 and l5 of the aforementioned Carpenter patent, the nal switches FSI and FSZ may be similar to the final switch shown in Fig. 61 of said Carpenter patent, the originating senders OSI .and OS2 may be similar to the subscribers register sender shown in Figs.

22 to 26 of said Carpenter patent, and the termi` nating senders TSI and TS2 may be similar to the terminating senders shown in Figs. 46 and 47 .of said Carpenter patent. lThe districtand t lunder ,the control of the marker.

final circuits IIB, IZB, I3!! and |40 are similar to the district circuit D-I shown in Fig. 5 of the patent to Hubbard-Matthies-Myers, No. 2,345,048 dated March 2S, 1944 except that each of applicants modulators Ilil, |24, i3d and Idd has a variable frequency carrier source associated therewith tunable under the control of anmoriginating marker and each of applicants demodulators H8, E28, I33 and M8 has a fixed frequency carrier source associated therewith; whereas in the Hubbard-Matthies-Myers patent the transmitter (modulator) T-- of the district `circuit D-i has a fixed frequency carrier source associated therewith and the receiver (demodulator) R5 of the district D-I is tunable Lunder the control of an originating marker to a desired channel frequency. Each of the originating markers OMI, CM2 and terminating markers TMI, 'PM2 is similar to the corresponding decoder markers of the aforementioned Carpenter patent except that it is arranged to control the tuning of the variable frequency carrier source of the modulator of a connected district or nal circuit in similar manner to that in which the originating marker OMI of the aforementioned patent to Hubbard-Matthies-Myers controls the tuning of the receiver R-5 of the district D-I shown in Fig. 5. In the aforementioned Carpenter patent, the originating decoder marker is shown in Figs. 29 to 40 and the terminating decoder is shown in Figs. 50 to 56. In the Hubbard- Matthies-Myers patent the originating marker OM is shown in Figs. l2 to 20.

Each district circuit and each final circuit is connected, through a low-pass filter, tc a voice frequency hybrid coil; and each hybrid coil is further connected to a line balancing network, to a modulator :for one direction of communication, and to a demodulator for the other direction of communication. 'I'he low-pass filter passes frequencies below 3,000 cycles per second 7,and the modulators are preferably of the doublebalanced copper-oxide type to obtain maximum linearity and thus avoid eXtra band products which would increase cross-talk. Each demodulator has a particular carrier source associated therewith and is connected to a multichannel medium through a band-pass filter tuned to pass a band, the cut-olf frequencies of which are equal to the carrier frequency of the associated demodulator plus and minus the voice frequency band. For instance, if the carrier frequency of a dernodulator is 100,000 cycles per second, the band-pass lter associated with the demodulator in question passes a channel band of 97,000 to 103,000

cycles per second. Thus there are as many channels through the multichannel media CMI I, CMIZ, CMI3 and CMM as there are demodulators connected thereto; and the carrier frequencies have at least an 8 kilocycle spacing. Each modulator is supplied with any desired one of a plurality of carrier sources, the connection of a desired carrier being under the control of the originating or terminating marker which is controlling the establishment of a desired conversa- A.

tional connection through the associated district or iinal circuit. As shown in Fig. l the marker tunes a variable frequency carrier source to the desired frequency; 'but this arrangement may be modified according to Fig. 1A in which a desired one of a plurality of carrier sources is selected Each modulator is connected through a pentode amplifier to one of the multichannel media, the inherent ineiiiciency of pentodes as plate modulator ampli- Illi() fiers being eifective to prevent cross-modulation between channels.

Assuming now that a call is initiated at the subscribers station SI, the line LI is extended through a line switch LSI to a district circuit I I and an originating register sender OSI is connected to the district, in the manner set forth in detail in the aforementioned Carpenter patent beginning on page 14, line'68, second column and `ending on page 26, line 3'7, first column. The number of the subscribers station, with which a conversational connection is desired, is dialed by the calling subscriber and is registered in the sender OSI, as described in the Carpenter patent beginning ,on page 32, line 5, nrst column and ending on page 36,'1ine 4l, iirst column. An originating marker OMI is associated with the sender and the number registered in the sender is decoded, in the manner set forth in the aforementioned Carpenter patent beginning on page 36, line 44, rst column and ending on page 40, line 38, first column.

The marker OMI, thereupon, makes a trunk test, in the manner set forth in detail in the aforementioned patent to Hubbard-Matthies- Myers beginning on page 3, line 71, second column and ending on page 5, line 46, second column, to select a trunk or channel through media CMI i and CMIZ which' is not in use and the demodulator of which in oiiice B isiassociated With a iinal switch having access to the line of the called station. The variable frequency carrier source IIE of the modulator II4, which is associated with the district H8, is tuned to the required frequency under the control of the' marker OM! in similar manner to that in which the receiver Rf-- of the district D-I in the Hubbard- Matthies-Myers patent is tuned to the required frequency as described in the Hubbard-Matthies- Myers patent on page 6, beginning in line 54, first column. A seizure signal is transmitted from the district circuit II through the low-pass filter lli, hybrid coil H2, modulator H4 and channel amplifier IIB, through common medium CMII, line amplifier LAI, common medium CMIZ, through the particular band-pass filter I lll which is tuned to pass the carrier wave and signal band transmitted by modulator I I4, to the demodulator Hi8 Whose carrier source |49 is of the same frequency as that of the source H5 connected to modulator IIfi. The demodulated seizure signal is thus further transmitted through hybrid coil 142 and low-pass lter MI to the nal circuit M0. In response Yto the seizure signal reecived by final circuit M, a terminating sender TS2 is connected to the final circuit |40. Numerical signals are now transmitted by the originating sender OSH, through district circuit IIII, low-pass filter lil, hybrid coil H2, modulator IIli, amplier HS, common medium CMII, line amplifier LAI common lmedium CMI'2, and band-pass filter Ill?, to demodulator M8; and the demodulated signals are transmitted through hybrid coil |42, filter yIlil and final circuit M0 to the terminating sender TS2, in which the identi-ty of the called subscribers station is thereby registered. For description of the sending of the seizure signal from the district H0, the connection of a terminating sender to the final circuit M0, and 'transmission of the numerical digits from the originating sender to the terminating sender, reference may be had to the aforementioned Hubbard-Matthies-Myers patent beginning on page 8, line 6, first column. The originating sender further transmits a signal indicative of the frequency of the carrier source' |I9 which is connected to the 'demodulator I I8 which is associated with hybrid coil I|2 and district III); and this signal' operates register means in Vsender TS2. A terminating marker TM2 is thereupon connected to the sender TS2,V the marker is connected to the modulator |44 and final switch FSZ which are associated with the nal circuit |40, the variable frequency lcarrier source Aof modulator |44 is tuned to the same frequency as that of the source I I9 associated with the demodulator I|8 in office A, and the nal switch F82 is 00ntrolled by the marker TM2 to connect with Vthe line L4 of the called station S4. The called line is tested and, if the line is idle, a signaling circuit is closed in the usual manner. Whenthe call is answered, a two-Way conversational connection iscompleted between stations SI and S4, one way through media CMI and CM|2 and the other way through media CMI3 and CMI4. The originating marker OMI is released as soon as the carrier source I I5 has been tuned to the required frequency; the originating sender OSI is released as soon as the numerical and channel frequency indicating signals havey been transmitted to th'e called office; and the terminating sender TS2 and marker TM2 are released as soon as connection with the called line has been completed. Supervisory signals maybe transmitted in both directions, release of the connection being Veffected when the subscriber at the calling station SI restores the telephone set to normal. The line and nal switches LSI and FSZ are thereupon restored to normal; and the carrier sources I I5 and |45 are disconnected from modulators II4 and |44.

'1n the oces C and D shown in Fig. 2, the district circuits 2I0 and 23|) and iinal circuits 22|] and 240 are individually connected through low-pass filters 2|I, 23|, 22| and 24| to hybrid coils 2I2, 232, 222 and 242. The hybrid coils are further connected to line balancing networks 2|3, 233, 223 and 243,110 modulators 2|4, 234, 224 vand 244, and through channel amplifiers 2 l 9, 239, 22S and 249 to demodulators 2I1, 231, 221 and 241. Fixed frequency carrier sources 218, 238,v 223 and 248 are connected to the demodulators; and carrier sources of desired frequency are selected and connected to the modulators under control of originating and terminating markers. The modulators are divided into groups, modulators 2I4 and 224 in office C beingconnected through individual isolating resistance pads 2|6 and 223` and'through group filter GFI to the multichannel transmission medium CM2 I and modulators 234 and 244 in oiiice D being individually connected through resistance pads 235 and 24B and through group lter GF3 to multichannel transmission medium CM23. The resistancepads and group filters are used instead of pentode amplifiers as in Fig. 1. The demodulators are also divided into groups, group filter GF2 connecting a group of demodulators 2I1, 221 to multichannel transmission medium CM24 and group Vfilter GF4 connecting a group of demodulatorsV 231, 241 to multichannel transmission medium CM22. YSince the energy from one modulator, which feeds back into another modulator to cause cross-talk, is inversely proportional to the square of the number of modulators connected in parallel, the cross-talk due to intermodulation between channels will not be substantial with large numbers of modulators connected to each group filter. The loss which is introduced by the isolating resistance pads is made up in the line ampliers. f

6 A' further modification introduced in the arrangement of Fig. 2 is the elimination of` the demodulator filters shown in Fig. 1, the balance of the demodulators necessary to prevent cross-talk being obtained by the use of double-balanced copper-oxide modulators. The most serious crosstalkwill be contributed by the third harmonic of the carrier modulating with energy approximately three times the frequency of the wanted signal; and this type of cross-talk is prevented by the group lters which pass a non-harmonically related group of carriers. The remaining serious distortion products involve the square and higher orders of the signal energy; and these are reduced by operating the demodulators at low level, single tube amplifiers being provided to raise the output volume of each demodulator to a satisfactory level.

In the arrangement shown in Fig. 3, a double stage of modulation is provided with single sideband transmission. Each district circuit 3|0 is connected by a low-pass filter 3|| to a voice frequency hybrid coil 312; and each final circuit 33D is connected by a low-pass filter 33| to a voice frequency hybrid coil 332. The hybrid coil 3I2 is further connected to a line balancing network 3|3, a primary modulator 3I4 and a demodulator 32|; and thehybrid coil 332 is further connected to a line balancing network 333, a primary modulator 334 and a demodulator 34|. The primary modulator 3|4 is connected to fixed-frequency carrier source 3|5and through a corresponding band-pass filter SI5 to a secondary modulator 3I1; and the primary modulator 334 is connected to a fixed-frequency carrier source 335 and through a corresponding band-pass filter 336 to a secondary modulator 331. The channels are divided into groups, the same primary carrier frequency being used for all of the primary modulators in the group and a different secondary carrier frequency being used for each secondary modulator. The particular frequency of the carrier source 3|8 which is connected to the secondary modulator 3|1 on a given call is controlled by the originating marker under whose control the desired conversational connection is being established; and the particular frequency of the carrier source 338, which is connected to the secondary modulator 331 on a given call, is controlled by the terminating marker under whose control the desired conversational connection is being established. The frequency of the carrier connected to each primary modulator of a group of channels is greater than the band-width passed 55 by the group lter; and the band-pass lter of each channel selects one of the side-bands of the primary carrier and passes it to the secondary demodulator. The frequency of the carrier source 3|8 which is selectively'associated with 60 the secondary modulator 3 1, lies within the bandwidth of the group iilter 3H! and determines the position of the modulated single side-band in the group band. And the frequency of the carrier source 338 which is selectively associated with 65 the secondary demodulator 331, lies-within the band-width of the group iilter 339 and determines the position of the modulated single side-band in v the group band. Since the unwanted'side band of each channel is outside of the group band and is not transmitted, onli7 a single stage of demodulation is required. Thus each group'of secondary modulators is, connected through a group filter 3|?! or 333 to a multichannel medium CMSI or CM33, thence through a line amplier, a multichannel medium CM32 or CM34, and an oice filter 32B or 34B to all of the demodulators 32! and 34|, each demodulator having a different xed carrier frequency. While this arrangement requires the use of an additional modulator and filter for each channel, it greatly reduces the cost of the required carrier supply system, reduces the delay distortion, and increases the number of channels which may be provided through the common transmission media.

The carrier channel equipments may be arranged so that fixed carrier frequencies are assigned to both the modulator and demodulator associated with a nal switch in which case the originating marker selectively controls the connection of carrier sources of required frequency toboth the modulator and demodulator associated with the district through which a call is initiated. This and other modifications of the above-described arrangements may be made Within the scope of the invention.

What is claimed is:

1. In a telephone system, subscribers lines, a plurality of high frequency carrier channel equipment units each comprising a modulator and a demodulator, means for individually connecting said units with subscribers lines, amplifiers, one for each of said modulators, band-pass filters, one for each demodulator, for passing a difierent band of frequencies to each demodulator, a plurality of carrier current sources, a source of a different frequency for each demodulator, a multichannel transmission medium connected to a plurality of said amplifiers, a multichannel transmission medium connected to a plurality of said filters, an amplier interconnecting said multichannel media, and means including control means common to said units for selectively connecting to the modulator of any unit connected with a calling line a carrier source of the same frequency as that of the carrier source connected to the demodulator of a unit adapted to be connected to the called line.

2. In a telephone system, subscribers lines, a plurality of high frequency carrier channel equipment units, means including electromechanical switching means for individually connecting said equipment units to subscribers lines, each of said units comprising a modulator and a demodulator, multichannel transmission media common to said units. a plurality of carrier current sources, a source of a different frequency for each of Said demodulators, and means including control means common to a plurality of said units for selectively effecting the connection of carrier sources of required frequency to said modulators, thereby to effect the completion of noninterfering two-Way conversational connections through said common media between units connected to calling subscribers lines and units adapted to be connected to called subscribers lines.

3. In a telephone system, subscribers lines, plurality of carrier channel equipment units, each comprising a modulator and a demodulator, means for individually connecting said units With subscribers lines, a multichannel transmission medium common to a plurality of saidv modulators, resistance pads one for each of said plurality of modulators, each of said pads connecting the associated modulator to said medium, a multichannel transmission medium common to a plurality of said demodulators, an amplifier interconnecting said media, a plurality of carrier current sources, a source of a different frequency for each of said demodulators, and means including control means common to said units for selectively connecting to the modulator of any unit connected to a calling subscribers line a carrier source of the same frequency as that oi the carrier source connected to the demodulator of a unit adapted to be connected to the called line.

4, In a telephone system, subscribers lines, a plurality of high frequency carrier channel equipment units, means for individually connecting said units With subscribers lines, each of said units comprising a modulator and a demodulator, a multichannel transmission medium common to a plurality of said modulators, resistance pads one for each of said modulatorsl connecting said modulators to said medium, a multichannel transmission medium common to a plurality of said democlulators, an amplifier interconnecting said media, a plurality of carrier current sources, a source of a different frequency for each of said demodulators, and means including control means common to said units for selectively connecting to the modulator of any unit adapted to be connected to a called subscribers line a carrier source of the same frequency as that of the source connected to the demodulator of the unit connected to the calling subscribers line.

5. In a telephone system, subscribers lines, a plurality of carrier channel equipment units, each comprising a modulator and a demodulator, means for individually connecting said units with subscribers lines, a multichannel transmission medium common to a plurality of said modulators, resistance pads one for each of said plurality of modulators, each of said pads connecting the associated modulator to said medium, a multichannel transmission medium common to a plurality of said demodulators, an amplifier interconnecting said media, a plurality of carrier current sources, a source of a different frequency for each of said demodulators, means including control means common to said units for selectively connecting to the modulator of any unit connected to a calling subscribers line a carrier source of the same frequency as that of the carrier source connected to the demodulator of a unit adapted to be connected to the called line and for selectively connecting to the modulator of said unit adapted to be connected to a called subscribers line a carrier source of the same frequency as that of the demodulator connected to the unit connected to the calling subscribers line.

6. In a telephone system, subscribers lines. a plurality of high frequency carrier channel equipment units, means for individually connecting said units with subscribers lines, each of said units comprising a modulator and a demodulator, said modulators and said demodulators being divided into groups, group filters, one for each group ofmodulators and one for each group of demodulators, resistance pads, one for each of said modulators, each of said pads connecting the associated modulator to the associated group filter, multichannel transmission media, the group lters of each o-f a plurality of groups of modulators being connected to one of said transmission media and the group filters of each of a pluraltiy of groups of demodulators being connected to another of said transmission media.

'7. In a telephone system, subscribers lines, a plurality of high frequency carrier channel equipment units, means for individually connecting said units with subscribers lines, each of said units comprising a modulator and a demodulator, said modulators and said demcdulators being divided into groups, group filters, one for each group of modulators and one for each group of demodulators, resistance pads, one for each of said modulators, each of said pads connecting the associated modulator tothe associated group filter, multichannel transmission media, the group filters of each of a plurality of groups of modulators being connected to one of said transmission media and the group lters of each of a plurality of groups of demodulators being connected to another of said transmission media, a plurality of carrier current sources, a source of a different frequency for each of said demodulators, and means including control means common to said units lfor selectively connecting to the modulator of any unit connected to a calling subscribers line a carrier source of the same frequency as that of the carrier sourcev connected to the demodulator of a unit adapted to be connected to the called subscribers line.

8. In a telephone system, subscribers lines, a

plurality of high frequency carrier channel equipment units, means -for individually connecting said units with subscribers lines, each of said units comprising a modulator and a demcdulator, said modulators and said demodulators being divided into groups, group filters, one for each group of modulators yand one for each group of demodulators, resistance pads, one for each of said modulators, each of said pads connecting the associated modulator to the associated group lter, multichannel transmission media, the group lters of each of a plurality of groups of modulators being connected to one of said transmission media and the group lters of each of a plurality of groups of demodulators being connected to another of said transmission media, a plurality of carrier current sources, a source of a diierent frequency for each of saidY demoduators, and means including control means common to said units for selectively connecting to the modulator of any unit, adapted to be connected to a called subscribers line a carrier source of the same frequency as that of the source connected to the demodulator of the unit connected to the calling subscribers line.

9. In a telephone system, subscribers lines, a plurality of high frequency carrier channel equip- `v ment units, said units divided into groups, means for individually connecting said units with subf scribers lines, each of said units comprising a primary modulator, a secondary modulator and a demodulator, multichannel transmission media common to a plurality of said units, group lters, each of said iilters connecting one of said media to the secondary modulator ofv a different group of said units, a plurality Aof sources of carrier current, a source of a different frequency being individually connected to each of said primary modulators and said demodulators, band-pass lters adapted to pass one of the voice frequency side-bands from the associated modulator to the associated secondary modulator, and means including control means common to said units for selectively connecting to the secondary modulator of a unit connected to a calling subscribers line a carrier source of required frequency to establish a conversational connection through one of said media with the demodulator of a unit adapted to be connected with the called subscribers line.

10. In a telephone system, subscribers lines, a plurality of high frequency carrier channel equipment units, said units divided into groups, means for individually connecting said units with subscribers lines, each of said units comprising a primary modulator, a secondary modulator and a demodulator, multichannel transmission media common to a vplurality of said units, group lters, each of said filters connecting one of said media to the secondary modulator of a different group of said units,l a plurality of sources of carrier current, a source of a different frequency being individually connected to each of said primary modulators and said demodulators, band-pass filters adapted to pass one of the voice frequency side-bands from the associated modulator to the associated secondary modulator, and means including control means common to said units for selectively connecting to the secondary modulator of a unit adapted to be connected to a called subscribers line a carrier source of the required frequency to establish a conversational connection through one of said media with the demodulator of the unit connected to the calling subscribers line.

RAYMOND O. WISE. 

